A significant percentage of patients with Crohn?s disease, a form of inflammatory bowel disease, have a mutation in CARD15 (NOD2) an intra-cellular protein that recognizes a breakdown product, muramyl dipeptide (MDP), of a common bacterial component, peptidoglycan (PGN). MDP via CARD15 signaling and its parent molecule, PGN via TLR2 (Toll-like Receptor 2) signaling activate the key intra-cellular inflammation switch, NF-kappaB. Thus, it was possible that the mutation disturbs the coordinate activation of NF-kappaB by these separate signaling pathways. In studies to explore this possibility, it was shown that mice lacking CARD15, manifest greatly increased IL-12, IL-18 and IFN-gamma production by monocytes when stimulated by PGN and that production of these cytokines by normal monocytes stimulated via TLR2 by TLR2 stimulants that do not contain MDP was inhibited by MDP. From this it was evident that MDP normally down-regulates PGN induction of inflammatory cytokines. In subsequent studies of the molecular basis of this down-regulation it was shown that CARD15 signaling leads to inhibition of PGN induction of NF-kappaB activation, particularly the activation of c-Rel, an NF-kappaB component closely associated with the production of IL-12. Finally, in studies to explore whether these mechanisms do indeed explain the effect of CARD15 mutations in Crohn?s disease: introduction of a CARD15 gene bearing the patient mutation into CARD15-deficient cells did not correct the abnormal IL-12 production of these cells whereas introduction of a normal CARD15 gene did lead to correction. Taken together, these findings reveal a new mechanism by which innate responses to bacterial products are regulated and show that an abnormality of this mechanism can lead to excess IL-12 production and the inflammation of Crohn?s disease.